In recent year, with the rapid development of information instruments, particularly with the advancement of portable terminal equipments, demand for display devices which are miniature, thin and of a low-consumption power type and which have such display capacity and display performance as in conventional CRT's has increased. In reply to the requirements of small size, small wall thickness and low power consumption, watches, portable electronic calculators and display terminal equipment of about 1/100 duty or less heretofore have been used in which the multiplex drive of a TN type liquid crystal cell having a twist angle of 90.degree. is utilized, but it is difficult in principle to further increase the duty number, because this leads to the deterioration of the display performace.
On the other hand, the supertwisted birefringence effect mode (hereinafter referred to simply as "SBE mode") utilizing the birefringence effect of a liquid crystal cell has been suggested in which the widest twist angle for providing bistable properties based on electro-optical characteristics of a chiral nematic liquid crystal is set so as to be in the range of 180.degree. to 270.degree.. T. J. Scheffer et al. have reported that the SBE mode can provide a matrix display which can drive as in the usual TN mode having a twist angle of 90.degree., that is, a matrix display which can drive by a fast scan addressing mode in response to the root-meansquare value of an applied voltage, and that the SBE mode has excellent contrast and a wider visual angle than the 90.degree. twisted TN display [see T. J. Scheffer and J. Nehring, Appl. Phys. Lett., 45, 1021 (1984)].
Characteristics necessary for the liquid crystal composition used in the SBE mode display element are as follows:
(1) Voltage-transmittance properties should be steep.
(2) The nematic-isotropic liquid phase transition temperature should be high.
(3) The suitable optical anisotropy value (hereinafter referred to simply as ".DELTA.n") should be obtained in accordance with the thickness (hereinafter referred to simply as "d") of a cell (.DELTA.n=n.sub.e - n.sub.o wherein n.sub.e is the refractive index of the extraordinary ray and n.sub.o is the refractive index of the ordinary ray).
(4) The viscosity should be low.
In the SBE mode, the above-mentioned characteristic (2) is necessary to inhibit a display from coloring owing to the dependence of .DELTA.n on temperature, and it is desired that the clearing point be as high as possible. The above characteristics (4) is particularly effective to shorten a response time in the liquid crystal cell having the SBE mode.
An object of the present invention is to provide a nematic liquid crystal composition having a high clearing point, a low viscosity, an ability to obtain a desired .DELTA.n, and an improved steepness of voltage-transmittance properties in the SBE mode display. Another object of the present invention is to provide a liquid crystal display element having a steep voltage-transmittance curve and good contrast in the SBE mode.